Minimum And Maximum Thresholds - Cisco 10000 Series Configuration Manual

Chapter 11 Managing Packet Queue Congestion
random-detect ip precedence
You must first enable the drop mode by using the random-detect prec-based command. You can then
set the drop probability profile by using the random-detect ip precedence command.
With the prec-based keyword, WRED uses the IP precedence value to calculate the drop probability.
For all precedence levels, the mark-probability-denominator default value is 10, and the max-threshold
is based on the output buffering capacity and the transmission speed for the interface.
If you want weighted random early detection (WRED) to ignore the precedence level when determining
which packets to drop, enter this command with the same parameters for each precedence level.
Remember to use reasonable values for the minimum and maximum thresholds.
random-detect discard-class
You must first enable the drop mode by using the random-detect discard-class-based command. You
can then set the drop probability profile by using the random-detect discard-class command.
With the discard-class-based keyword, WRED uses the discard-class value to calculate the drop
probability.
random-detect cos
You must first enable the drop mode by using the random-detect cos-based command. You can then set
the drop probability profile by using the random-detect cos command.
With the cos-based keyword, WRED uses the cos bit value to calculate the drop probability.

Minimum and Maximum Thresholds

The random-detect command allows you to specify the minimum and maximum threshold settings for
a class queue:
•
•
Table 11-7
DSCP, IP precedence, and discard-class. For example, if a user-defined drop profile is not available, for
discard-class 3, the router calculates the minimum and maximum thresholds as follows:
The drop probability indicates that the router drops one packet for every 10 packets.
Table 11-7
Note
queue, the same profile applies to all DSCP or precedence values. If you do not configure the default
profile, the behavior is to tail drop.
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Minimum threshold—The minimum number of packets allowed in the queue. When the average
queue length reaches the minimum threshold, weighted random early detection (WRED) randomly
drops some packets with the specified DSCP, IP precedence, discard-class, or atm-clp value. Valid
minimum threshold values are from 1 to 16,384.
Maximum threshold—The maximum number of packets allowed in the queue. When the average
queue length exceeds the maximum threshold, WRED drops all packets with the specified DSCP, IP
precedence, discard-class, or atm-clp value. Valid maximum threshold values are from the value of
the minimum threshold to 16,384.
lists the default drop thresholds for weighted random early detection (WRED) based on
Minimum threshold = 11/32 * queue size
Maximum threshold = 1/2 * queue size
applies to the PRE2. On the PRE3, when you specify a WRED default drop profile for a
Controlling Congestion Using Weighted Random Early Detection
Cisco 10000 Series Router Quality of Service Configuration Guide
11-19